The present invention relates generally to a staged combustion system in which the production of undesirable combustion product components is minimized over the engine operating regime and, in particular, to a combustion system having a plurality of free floating mixer assemblies which are independently retained in position with respect to a corresponding opening in the dome plate in a manner so as to be prevented from rotating while being movable in a radial and axial direction.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Aircraft are govemed by both Environmental Protection Agency (EPA) and International Civil Aviation Organization (ICAO) standards. These standards regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) from aircraft in the vicinity of airports, where they contribute to urban photochemical smog problems. Such standards are driving the design of gas turbine engine combustors, which also must be able to accommodate the desire for efficient, low cost operation and reduced fuel consumption. In addition, the engine output must be maintained or even increased.
It will be appreciated that engine emissions generally fall into two classes: those formed because of high flame temperatures (NOx) and those formed because of low flame temperatures which do not allow the fuel-air reaction to proceed to completion (HC and CO). Balancing the operation of a combustor to allow efficient thermal operation of the engine, while simultaneously minimizing the production of undesirable combustion products, is difficult to achieve. In that regard, operating at low combustion temperatures to lower the emissions of NOx can also result in incomplete or partially incomplete combustion, which can lead to the production of excessive amounts of HC and CO, as well as lower power output and lower thermal efficiency. High combustion temperature, on the other hand, improves thermal efficiency and lowers the amount of HC and CO, but oftentimes results in a higher output of NOx.
One way of minimizing the emission of undesirable gas turbine engine combustion products has been through staged combustion. In such an arrangement, the combustor is provided with a first stage burner for low speed and low power conditions so the character of the combustion products is more closely controlled. A combination of first and second stage burners is provided for higher power output conditions, which attempts to maintain the combustion products within the emissions limits.
Another way that has been proposed to minimize the production of such undesirable combustion product components is to provide for more effective intermixing of the injected fuel and the combustion air. In this way, burning occurs uniformly over the entire mixture and reduces the level of HC and CO that results from incomplete combustion. While numerous mixer designs have been proposed over the years to improve the mixing of the fuel and air, improvement in the levels of undesirable NOx formed under high power conditions (i.e., when the flame temperatures are high) is still desired.
One mixer design that has been utilized is known as a twin annular premixing swirler (TAPS), which is disclosed in the following U.S. Pat. Nos. 6,354,072; 6,363,726; 6,367,262; 6,381,964; 6,389,815; 6,418,726; 6,453,660; 6,484,489; and, 6,865,889. Published U.S. patent application 2002/0,178,732 also depicts certain embodiments of the TAPS mixer. It will be understood that the TAPS mixer assembly includes a pilot mixer which is supplied with fuel during the entire engine operating cycle and a main mixer which is supplied with fuel only during increased power conditions of the engine operating cycle. While improvements in NOx emissions during high power conditions are of current pinmary concern, modification of the main mixer in the assembly is needed to maintain the mixer assembly in proper position.
It is well known within the combustor art of gas turbine engines that a dome portion, in conjunction with inner and outer liners, serves to form the boundary of a combustion chamber. The annular combustor dome also serves to position a plurality of mixers in a circumferential manner so that a fuel/air mixture is provided to the combustion chamber in a desired manner. While the typical combustor arrangement has adequate space between swirler cups to incorporate features to enhance the spectacle plate structure (e.g., the addition of ribs, cooling holes and the like), certain geometric restrictions have been introduced by current combustor designs utilizing the TAPS mixer. As disclosed in U.S. Pat. No. 6,381,964 to Pritchard, Jr. et al., the size of the fuel nozzle and the corresponding swirler assembly associated therewith, has increased significantly from those previously utilized and thereby reduced the distance between adjacent swirler cups. Utilization of an annular dome plate having a greater diameter would serve to increase the weight of the engine and require modification of components interfacing therewith. Thus, the openings in the dome plate have been enlarged and thereby lessened the circumferential distance between adjacent openings.
One combustor dome assembly design including a floating swirler is disclosed in a patent application entitled “Combustor Dome Assembly Of A Gas Turbine Engine Having A Free Floating Swirler,” having Ser. No. 10/638,597, which is owned by the assignee of the present invention. As seen therein, tab members are associated with the outer and inner cowls to restrict radial and axial movement of the swirlers to a predetermined amount. Alternatively, separate tab members are provided which interface with the connections of the dome plate, liners and cowls. While such tab members are able to perform their intended function, their positioning upstream of the swirler is not practical for the mixer assembly of the current design.
In yet another known combustor dome assembly, anti-rotation tab members for a mixer assembly are located only on the mixer itself and interface with the tab members of mixer assemblies located adjacent thereto. It has been found that this configuration is subject to an offset between the mixer assembly and the corresponding opening in the dome plate, which may be caused by vibrations experienced by the adjacent mixer assemblies or machining errors. Further, cooling holes in the radial section of the dome plate between adjacent openings tend to be obstructed, which has increased the temperature of the deflector plate located downstream thereof to by an amount that has affected the life of the deflector plate.
Accordingly, it would be desirable for a mechanism to be developed in association with the current dome and mixer assembly design which prevents rotation of the mixer assembly. It would also be desirable for such mechanism to permit the mixer assembly to have a predetermined amount of axial and radial movement.